Revista Colombiana de Matem´aticas Volumen 46(2012)2, p´aginas113-144 On the Classification of 3{Bridge Links Sobre la clasificaci´onde los enlaces de 3 puentes Hugh Michael Hilden1, Jose´ Mar´ıa Montesinos2, Debora´ Mar´ıa Tejada3;a, Margarita Mar´ıa Toro3;a;B 1University of Hawaii at Honolulu, Hawaii, USA 2Universidad Complutense de Madrid, Madrid, Espa~na 3Universidad Nacional de Colombia, Medell´ın,Colombia Abstract. Using a new way to represent links, that we call a butterfly repre- sentation, we assign to each 3-bridge link diagram a sequence of six integers, collected as a triple (p=n; q=m; s=l), such that p ≥ q ≥ s ≥ 2, 0 < n ≤ p, 0 < m ≤ q and 0 < l ≤ s. For each 3-bridge link there exists an infinite num- ber of 3-bridge diagrams, so we define an order in the set (p=n; q=m; s=l) and assign to each 3-bridge link L the minimum among all the triples that corre- spond to a 3-butterfly of L, and call it the butterfly presentation of L. This presentation extends, in a natural way, the well known Schubert classification of 2-bridge links. We obtain necessary and sufficient conditions for a triple (p=n; q=m; s=l) to correspond to a 3-butterfly and so, to a 3-bridge link diagram. Given a triple (p=n; q=m; s=l) we give an algorithm to draw a canonical 3-bridge diagram of the associated link. We present formulas for a 3-butterfly of the mirror image of a link, for the connected sum of two rational knots and for some important families of 3-bridge links. We present the open question: When do the triples (p=n; q=m; s=l) and (p0=n0; q0=m0; s0=l0) represent the same 3-bridge link? Key words and phrases. Links, 3-bridge links, Bridge presentation, Link dia- gram, 3-butterfly, Butterfly presentation. 2010 Mathematics Subject Classification. 57M25, 57M27. Resumen. Usando una nueva forma de representar enlaces, que se denomina representaci´onen mariposa, se asocia a cada diagrama de 3 puentes de un en- lace una sucesi´onde seis enteros, organizados como una tripla (p=n; q=m; s=l), a Partially Support by Colciencias, code 1118-521-28160. 113 114 H. M. HILDEN, J. M. MONTESINOS, D. M. TEJADA & M. M. TORO tal que p ≥ q ≥ s ≥ 2, 0 < n ≤ p, 0 < m ≤ q y 0 < l ≤ s. Para cada enlace de 3 puentes existe un n´umeroinfinito de diagramas de 3 puentes, por lo que se define un orden en el conjunto de triplas de la forma (p=n; q=m; s=l) y se asigna a cada enlace de 3 puentes L el m´ınimoentre todas las triplas que corresponden a una 3-mariposa de L, y que se llama la presentaci´onen mari- posa de L. Esta presentaci´onextiende, en una forma natural, la bien conocida clasificaci´onde Schubert de los enlaces de 2 puentes. Se obtienen condiciones necesarias y suficientes para que una tripla de la forma (p=n; q=m; s=l) corresponda a una 3-mariposa y por tanto, a un dia- grama de 3 puentes de un enlace. Dada una tripla (p=n; q=m; s=l) se da un algoritmo para dibujar, en forma can´onica, un diagrama de 3 puentes del en- lace de 3 puentes asociado. Se presentan f´ormulas para la 3-mariposa de la imagen espejos de un enlace de 3 puentes, para la suma conexa de dos nudos racionales y de algunas familias importantes de enlaces de 3 puentes. Queda la pregunta abierta: ¿Cu´andodos triplas (p=n; q=m; s=l) y (p0=n0; q0=m0; s0=l0) representan el mismo enlace de 3 puentes?. Palabras y frases clave. Enlaces, enlaces de 3 puentes, presentaci´onen puentes, diagrama de enlace, 3-mariposa, presentaci´onen mariposa. 1. Introduction Our goal in this paper is to study 3-bridge link using a new presentation of links, called butterfly, that we introduced in [6], [5], [4], [7] and [8]. Up to now, the classification of 3-bridge links has not been realized, and it presents difficulties that contrast with the case of 2-bridge links, that were completely classified by Schubert, [19]. We will associate to each 3-bridge link diagram a set of 6 integers, extending the classification of 2-bridge links given by Schubert, [19]. As each 3-bridge link admits different 3-bridge diagrams, (in fact, infinitely many), the situation is more difficult and we can not expect the same type of classification that was obtained for 2-bridge links. In Section 2 we present a brief review of the concept of m-butterfly and some results from [7] and [8]. The main result in [7], that the butterfly number coincides with the bridge number, is the starting point for our work: to study the 3-butterflies with the objective of classifying the 3-bridge links. For our purpose, we need to consider only reduced 3-bridge link diagrams, so we will impose restrictions on the type of butterfly we will consider. In Section 3 we assign a set of positive integers fp; n; q; m; s; lg to each 3-butterfly, that we write as a triple (p=n; q=m; s=l) for geometrical reasons, to be explained in that section. We obtain necessary and sufficient condi- tions for a triple (p=n; q=m; s=l) to correspond to a 3-butterfly and so to a 3-bridge link. This is done by defining a permutation µ associated to each triple (p=n; q=m; s=l) and studying its cyclic decomposition. This cyclic decom- position answers the question of the number of components of the link diagram Volumen 46, N´umero2, A~no2012 ON THE CLASSIFICATION OF 3{BRIDGE LINKS 115 associated to the 3-butterfly (p=n; q=m; s=l) and provides an algorithm to draw a canonical link diagram associated to it. We define the concept of butterfly equivalence and find some conditions for two 3-butterflies to be equivalent. It is an open problem to find necessary and sufficient conditions for the equiva- lence of two 3-butterflies and to find a complete set of \moves" to transform a 3-butterfly into an equivalent one. All the constructions and algorithms are easily implemented, and we have done so using the software Mathematica.A variation of the permutation µ allows us to find the combinatorial knot (or Gauss code) associated to a link diagram given by (p=n; q=m; s=l). Then it is possible to compute invariants of the link, such as the link group or an invariant polynomial. See [24], [17] and [23] for details on combinatorial knots. In Section 4 we define the butterfly representation of a link L. We do so by ordering the set of all 3-butterflies and taking the minimum, in that order, of all 3-butterflies that represent the link L. In Section 5 we give a 3-butterfly representation of two basic link construc- tions: the mirror image of a 3-bridge link and the connected sum of two rational knots. Also, we give a 3-butterfly representation of some families of 3-bridge links: the pretzel P (a; b; c), the torus link T (p; 3) and the 3-bridge knots up to 9 crossings in Rolfsen's table [18]. We end this section by giving a 3-butterfly interpretation of the constructions given in [2], [12] and [13]. In the last section we present some concluding remarks regarding the open problems that arise and the applications of our construction. The basic infor- mation on links and knots can be found in [1], [3] or [14]. 2. About n-Butterflies Intuitively, an m-butterfly is a 3-ball B3 with m > 0 polygonal faces on its 2 3 boundary S = @B , such that each face C is subdivided by an arc tC in two subfaces (that have the same number of vertices) that are identified by a “reflection” along this arc tC . Thurston's construction of the borromean rings, [21] and [22], is a nice example of this construction, that we generalize for all links in [7]. In this example we notice that the cube is actually a closed 3-cell B3, with twelve faces on its boundary that are identified by reflections along some axes. Moreover, pasting the faces of the cube we obtain S3 and the set of axes become the borromean rings. In terms of butterflies, that we are going to define in Section 2.1, the cube edges and vertices form the graph and the axes form the trunk of a 6-butterfly, (see Figure 1a). In this section we give the definitions and basic aspects of the constructions given in [7], and in the rest of the paper we will consider the case m = 3. For more details and proofs see [7]. Revista Colombiana de Matem´aticas 116 H. M. HILDEN, J. M. MONTESINOS, D. M. TEJADA & M. M. TORO 2.1. Definitions and Constructions Let R be a connected graph embedded in S2 = @B3, where B3 is a closed 3-cell, so that S2 − R is a disjoint union of open 2-cells. We denote each open 2-cell generically by C. For any n 2 N, let P2n be the regular polygon that is the closed convex hull of the 2nth roots of unity. We define a parametrization of C to be a function f from P2n to the closure C of C, with the following properties: a) The restriction of f to interior P2n is a homeomorphism from interior P2n to C.